Place map generator spawns more intuitively

The map generators previously placed spawns in an order that was
somewhat arbitrary and unintuitive to users.

This change provides and makes use of a Terraformer utility to reorder
spawns, based on their location, so that they typically make more
sense to users selecting spawns from a lobby.

Added to ExperimentalMapGenerator (RA, CnC), and D2kMapGenerator.

Note that as this changes the output of the map generator for a given
selection of settings. (Replays using generated maps from older
versions will lose compatibility and no longer load.)

OpenRA.Mods.Common/Traits/World/ExperimentalMapGenerator.cs

@@ -947,6 +947,7 @@ namespace OpenRA.Mods.Common.Traits
 			// Cosmetically repaint tiles
 			terraformer.RepaintTiles(repaintRandom, param.RepaintTiles);
 
+			terraformer.ReorderPlayerSpawns();
 			terraformer.BakeMap();
 
 			return map;